When driving a vehicle at night, or in poor visibility conditions, the insufficient illumination generated by a vehicle's headlights and/or any ambient road lighting, nearly always limits the driver's visibility range. When driving, the driver needs a sufficient range, hence time, to identify a danger from a distance and to react accordingly. The range of vision an alert driver requires to escape danger may be calculated empirically. It is customary to calculate the range required by multiplying the driving speed by the time needed for the combined human and vehicle reaction time. This time may range from 6 to 10 seconds for highway driving to more than double for heavy off-highway vehicle, and even ten times that for trains, boats or landing aircrafts. For example, a driver cruising at a speed of 120 km/h (i.e., 33 m/sec), would need a visibility range of approximately 200-333 meters.
However, the light system of vehicles, even when the headlamps are set to the high beam lights state, will generally illuminate a distance not exceeding approximately 200 meters, i.e. approximately 6 seconds of driving at a speed of 120 km/h. However, It should be noted that the use of high beam lights or alternative high powered lights is not neither customary nor allowed for continuous driving, as it causes glare (“blinding”) other drivers. The situation is worse when the headlights are set to their low beam state, as the illuminated distance will not exceed 100 meters, generally, i.e. about only 3 seconds of driving at a speed of 120 km/h. The situation is worst during off-highway driving in a dusty environment where the visibility is down to 50 meters and time to stop may be 15-20 sec, or a boat sailing inside a fogy port with 50 meters visibility and time to react of 30-60 sec.
This seems to mean that drivers are driving currently under conditions of insufficient visibility range or, at least, in a state on the borderline of the required range of safe vision, and are, therefore, endangering themselves and others. Simply improving vehicle headlights to increase their illumination range may not be a suitable solution, because blinding of other drivers should be avoided, and also due to technological limitations.
With this current state of affairs, and as a natural outcome, developments in the field of night imaging equipment in aerospace/military applications have given rise to the idea of introducing and integrating night imaging systems in vehicles, which might increase the driver's visibility range. Furthermore, display systems for images received from observation systems under conditions of deficient vision or low visibility have also improved. Outstanding examples thereof are the HUD (Head Up Display) and LCD (Liquid Crystal Display) Systems.
One technology, which might be integrated and operated in vehicles, is the thermal imaging technology. An array of detectors, sensitive to infrared radiation absorbs the heat energy emitted by bodies and creates a video image according to the absorbed heat emission. An example of thermal imaging technology is described in U.S. Pat. No. 5,414,439, titled “Head Up Display with Night Vision Enhancement”. This patent describes the installation of an infrared camera in a vehicle for watching road conditions by using thermal imaging. The camera transmits a video signal to the HUD System, which displays the image on the vehicle's windshield (or any other optical combiner) located in front of the operator.
The system described in the '439 patent is based on a thermal camera, which does not offer a solution for perceiving differences in colors and shades of gray (a thermal camera displays heat distinctions and emissivity only). The system is heat sensitive and therefore, does not detect elements' temperature if equal to the road background temperature. In other words: the system does not offer the driver any help in reading signs, signposts, etc. Nor does the thermal camera exploit the extensive use of reflecting colors applied in road markings, light reflectors of vehicles, and so on. Furthermore, the thermal camera does not display the visible light (it operates in the remote infrared range). In addition, inasmuch as the system is based on an array of IR detectors operating on the remote infrared wavelengths (8 μm to 14 μm), it may not be possible to install the system inside a vehicle's driver compartment, because vehicle windows do not transmit energy in the remote IR wavelength.
Consequently, in considering an installation in a vehicle for the purpose of increasing the driver's range of vision, the disadvantages of thermal imaging technology are markedly apparent. The system is cumbersome, sensitive to various driving states, might not give a clear view of “cold” road signs, and is relatively expensive.
Another technology, which might apparently be integrated and assimilated in a vehicle, is the image intensification technology. An intensifier intensifies the available photons at the input by 50.000 to 100.000 times and enables observations in even a very dark environment. Intensification technology, however, is prone to “bloom”. The image intensifier is sensitive to photons of the visible and near IR wavelengths. Consequently, an image intensifier may be bloomed by sources in the visible wavelength and the near IR entering the field of view.
Therefore, for installation in a vehicle for the purpose of increasing the driver's visibility range, this technology also has marked disadvantages. A vehicle moving on the road will, of necessity, encounter light from oncoming, preceding, and passing vehicles, as well as from street lamps. These sources emit light in both the visible and near IR spectrum. These sources might cause intensifier saturation and bloom the provided image.
An additional imaging technology that may be used for nighttime imaging is gated imaging technology. To reduce the influence of interference in the space between the night imaging system and the target, gated imaging is used, such that energy reflected from the illuminated target is absorbed only in specified time intervals. In this manner, the image displayed may be influenced by the imaging receiver input only when the reflected energy from the illuminated target actually reaches it (after having covered, at the speed of light, the distance from the target to the imaging receiver). While the illumination's beams travel the distance to the target, and while the reflected energy (beams) from the target and its adjacent environment travel the distance from the target to the imaging receiver, the imaging receiver is switched “OFF”.
For application in vehicular installation aiming at increasing the driver's visibility range, the disadvantage of adding a source of light is apparent. For example, such technology may concern a system that requires radiation safety (e.g.—meeting the requirements of MPE standards) and the additional source of light should be at eye safety levels.
Night driving necessitates an increase in the driver' visibility range by implementing a system that might provide the driver with an expanded range of observation (e.g.—up to 500 meters ahead). Such a system may be required to operate in an environment saturated with sources of light in the visible and near IR wavelengths (e.g., headlights of other cars, roadside lights, other active night vision systems), while overcoming the challenge of eliminating blinding resulting from such sources of light, and without encountering radiation and other safety problems in influencing the system.